20XT CVT Primary Bowl

Stock CVTech Primary

Our cars use a continuously-variable transmission (CVT) between the engine and final drive reduction. Using a system of flyweights, ramps, and springs to adjust the distance between sheaves, it automatically shifts continuously to optimize engine power output.

Due to the mechanical complexity of these systems, we have historically opted for off-the-shelf solutions. In recent years, we have used a CVTech CVT due to its high shift range.

The CVTech is rated for a 22 HP engine at 6800 rpm, and the engine we must use for Baja is 10 HP at a maximum of 3800 rpm. This means that the CVTech is substantially overdesigned for what we need, and there is significant lightweighting potential. This weight saving is especially valuable, as it is a component with a very high rotational inertia on the fastest spinning point in the drivetrain.

The part of the primary that I chose to focus on is the "bowl," an upside-down bowl shaped part that the flyweights react and slide against as the CVT shifts out. At 3800 rpm, the bowl can see up to 2000 pounds of force from the flyweights. 

The stock bowl is made of steel, and weighs 1.62 lb. I set a goal to reduce weight by 50% by making it out of aluminum.

My bowl, which is machined 2024 aluminum, weighs only 0.60 lb - a 63% weight reduction. In acceleration testing over a 100-foot straight line, this bowl reduces the time by an average of 0.13 seconds (about 3.1%), which gives us a significant competitive edge.

The bowl has helped propel the JH20XT to our 5th overall season placement for the 2024 Baja SAE competition.

I used an Ansys simulation to analyze this part, ensuring that stresses were below the allowable (which was lowered due to the high operating temperatures of this part).


I programmed, set up, and operated the CNC mill to machine this part. This part was made in three steps. First, the inside of the bowl was milled, making flats to clamp onto in the second operation and boring through the stock to have a hole to indicate off of. Then, the part was flipped, and the exterior was machined, leaving the part on a base. This base was then cut off on the wire EDM.